Got to level 13. Very coolI
I found that if you have "Free Refill" "Piercing Bullets" "Bouncing bullets" "Shoots 9 bullets x times" you can spam them and noone is getting past
Still there as "refill energy to full". Just had an absolutely busted run with [randomize card cost] & [regen health] perks, and [refill energy to full], [shoot for each time played], [shoot 6 bouncing bullets] for filler, and mostly free-cost cards from the first perk. Got bored of the build at lvl35 after the last dozen waves were handled with QWER spamming & waving the mouse every now and then.